A team of US scientists has created two new types of biocompatible micro/nanomotor that harvest energy for their propulsion from the surrounding environment – overcoming a major obstacle to using many existing technologies for in vivo applications such as drug delivery, biopsies and precision nanosurgery.

Various types of motor have come to the fore over the past decade, some fueldriven and some fuel-free. However, fuel-driven motors commonly rely on hydrogen peroxide, hindering their use for in vivo applications, while many fuel-free versions are not biocompatible and rely on complex magnetic or electronic equipment to guide them around the body.

Wei Gao, a graduate student in Professor Joseph Wang’s lab at the University of California in San Diego, has published proof-of-concept results of a tubular microrocket made from zinc and two types of spherical micromotor made from aluminium, one of which is alloyed with gallium, the other part-coated in palladium, and both part-coated in titanium.

The zinc microrockets are 10μm long and are intended for use in the stomach, where the zinc reacts with hydrochloric acid to produce a stream of hydrogen bubbles for propulsion. They are fabricated using template electrodeposition, with an outer polyaniline (PANI) tube first polymerised inside the membrane template, then a zinc layer deposited galvanostatically within the PANI layer to form a bi-layer tube.

Gao says, ‘Millions of these microrockets can be fabricated from a single piece of membrane, so their fabrication can easily be scaled up’. He adds that they should be in real-world use in about five to 10 years’ time, once practical in vivo studies of targeted drug delivery have been successfully completed.

The aluminium-gallium (Al-Ga) motors are also driven by hydrogen bubbles, but in this case they are produced by the reaction between the aluminium and water. Since the human body is 70% water, this opens up a wide range of possible clinical applications.

Offering an even wider range of potential applications, though, is the aluminium-palladium (Al-Pa) type motor, which can switch autonomously between three types of fuel – acids, bases or hydrogen peroxide – depending on its surroundings. This is due to the hydrogen-producing reaction between aluminium, acids and bases, and the oxygen-producing reaction between palladium and hydrogen peroxide, and could be used for industrial, non-clinical settings, such as oil spill clean-ups and security applications.

The Al-Ga motors are fabricated by spreading aluminium particles measuring about 20μm in diameter and liquid gallium onto separate glass slides at 1:1 mass ratio. The slides are then pressed together to form the Al-Ga alloy by microcontact mixing and then separated, leaving the Al-Ga particles on one of them. The exposed hemisphere of the particles are coated with titanium using electron beam evaporation. To fabricate the aluminium palladium motors, the process is broadly the same, only here the aluminium particles are part-coated with titanium and then palladium.

Gao adds, ‘Both [motor types] are based on commercial aluminium microspheres, so they can easily be mass-produced, although the Al-Ga alloy formation process still needs to be optimised for this’.

The next phase of research will look at improving hydrogen bubble yield for both types (currently less than 10% for the Al-Ga type) to extend lifetimes, which are currently up to about 10 minutes.